Amphiphilic diblock copolymers comprising poly(methyl methacrylate-co-spirobenzopyran methacrylate) and poly(ethylene glycol) were synthesized using atom transfer radical polymerization to develop a new photoresponsive drug carrier. The diblock copolymers formed core-shell polymeric micelles with photoresponsive inner cores. Spirobenzopyran in the core was isomerized to the merocyanine form by UV irradiation, which resulted in reduction of the micellar size without decrease in the association number. This inferred that the strongly intermolecular interactions between merocyanine residues caused the conformational change of the core segments, following reduction of the micellar size.